1) Field of the Invention
This invention relates in general to a process for the preparation of mixed parylene dimers. In one aspect, this invention is directed to a process for the preparation of a mixture of parylene dimers which are free of alpha-halogens and wherein the dimer distribution in the mixture imparts desirable properties to parylene films and coatings prepared therefrom. In a further aspect, this invention relates to a process for preparing a mixture containing parylene dimers which are free of alpha-halogens and thereby avoid the environmentally undesirable formation of acid halides during deposits of parylene films and coatings. In another aspect, the invention is directed to compositions comprised of a blend of various dimers.
2) Background of the Related Art
Parylene is a generic term applied to a class of poly-p-xylylenes which are derived from a dimer of the structure: ##STR1##
Parylene is an inert, transparent, conformal coating which has excellent barrier properties and can be used at relatively high temperatures. Due to its ability to provide thin films and conform to substrates of varied geometric shapes, it is ideally suited for use as a conformal coating in a wide variety of fields, particularly in the electronics industry.
The term "parylene" as employed throughout the specification and appended claims is intended to encompass not only those products prepared by the vapor deposition of the p-xylylenes of formula (I) above, but other known parylenes prepared from p-xylylenes containing one or more substituents on the aliphatic or aromatic portion of the molecule.
The preparation of p-xylylene polymers by various routes has been reported in the patent literature. For example, U.S. Pat. No. 2,719,131 which issued in Sep. 27, 1955 to E. I. DuPont de Nemours and Company disclosed a process for preparing poly-p-xylene wherein the vapors of p-xylylene were pyrolyzed in the presence of chlorine gas.
Also in British patent number 650,947 which was granted Mar. 7, 1951, polymer formation was detected on the walls of a cooling chamber after p-xylylene was vaporized and pyrolyzed.
In U.S. Pat. No. 3,149,175 which issued Sep. 15, 1964 a process was reported for the preparation of di-para-xylylenes in yields of 10 percent and higher. The process involved pyrolyzing a mixture of steam and p-xylene at a temperature between about 800.degree. C. and 1000.degree. C. to generate a free radical and condensing the reactive diradical in a fluid medium.
The preparation of para-xylylene polymers was also disclosed in U.S. Pat. No. 3,342,754 which issued on Sep. 19, 1967 to W. F. Gorham and is assigned to Union Carbide Corporation. In this patent it is indicated that true linear homopolymers of para-xylylene could be produced in nearly quantitative yields by heating a cyclo-di-para-xylylene having up to 6 aromatic nuclear substituent groups to a temperature between about 450.degree. C. and 700.degree. C. for a time sufficient to cleave substantially all of the di-para-xylylene into vaporous para-xylylene diradicals and cooling the vaporous diradicals to a temperature below the condensation temperature.
Although a wide variety of methods have been reported in the literature for the preparation of parylene dimers, there have been no reports in the prior art relative to the use of mixtures or blends of different dimers which are entirely free of alpha-halogens and wherein the physical and/or chemical properties of parylene films and coatings can be varied.
Prior to the present invention when parylene coatings, films and the like were prepared, one would merely prepare the individual dimers, purify them and select the individual dimer which gave the properties closest to that desired and then utilize the starting compounds and deposition conditions most suitable for that particular dimer. No effort had been made to prepare a dimer mixture or blend having a predetermined distribution of non-halogenated, partially halogenated and fully halogenated paracyclophanes in order to control or vary the properties of the resulting coating or film.
It has been observed, however, that the properties, composition and deposition conditions of parylene films and coatings prepared from dimers, will vary depending upon the particular 2,2-paracyclophane dimer employed. Thus, in the past, 2,2-paracyclophane, monochloro-2,2-paracyclophane, dichloro-2,2-paracyclophane and tetrachloro-2,2-paracyclophane when subjected to vapor deposition conditions, each gave films and coatings having a variety of chemical and physical properties. Although it had been possible to prepare mixtures or blends of dimers of the substituted and unsubstituted 2,2-paracyclophanes, prior to the present invention such mixtures had not been prepared by the Hofmann elimination of a mixture of quaternary ammonium salts.
It has been noted that the preparation of dimers containing ring halogens by the chlorination of paracyclophane usually results in halogenation of at least some of the aliphatic carbon atoms. When these halogenated dimers are subjected to vapor deposition conditions for the formation of parylene films and coatings, the less firmly bonded aliphatic halogens are released and can form acid halides which require environmentally safe methods of disposal.
It is therefore an object of the present invention to provide a process for the preparation of mixed parylene dimers which are useful for the preparation of parylene films and coatings. Another object is to provide a process for preparing a mixture of halogenated and non-halogenated dimers of 2,2-paracylophanes wherein the distribution of monohalo- polyhalo- and unsubstituted dimers can be varied so as to alter the properties of the resulting films and coatings. A further object of the present invention is to provide a method for the preparation of blends of halogenated and non-halogenated paracyclophanes which are free of alpha-halogen substitution on the aliphatic carbon atoms.
In another object, the invention provides blends and mixtures of halogenated paracyclophanes containing halogen atoms only on the ring carbon atoms. A still further object is to provide an environmentally safe method for the vapor deposition of parylene from blends of halogenated paracyclophanes, which method avoids the formation of acid halides. A further object of the present invention is to provide a process for preparing the mixture of dimers which avoids the preparation and purification of the individual dimers. A further object is to provide articles coated with parylene which have been prepared by the vapor deposition of the dimer mixture. Another object of the invention is to provide a process for preparing a mixture of halogenated and nonhalogenated dimers of 2,2-paracyclophanes wherein the distribution of monohalo, polyhalo and unsubstituted dimers can be varied so as to alter the properties of the resulting parylene films and coatings. Another object of the invention is to provide a process which is simple and efficient and hence is effective in reducing the overall cost in the preparation of the paracyclophane dimer mixtures or blends. A further object of this invention is to provide a process for the preparation of the dimer mixture which provides even higher yields by utilizing dimethylsulfoxide in combination with a selected class of reaction promoters. These and other objects will readily become apparent to those skilled in the art in the light of the teachings herein set forth.